Method and apparatus electrostatically classifying toner particles

ABSTRACT

Method and apparatus to electrostatically classify electroscopic toner particles according to size. Particles to be classified are supported on a photocondctive surface and electrically charged to one polarity. An electrically conductive surface spaced a predetermined distance from the charged particles is electrically biased by a potential of opposite polarity to collect toner particles which are attracted to the surface in response to the magnitude of the biasing voltage and the between-surface spacing.

United States Patent Robinson 45 Ma 16, 1972 [54] METHOD AND APPARATUS476,991 6/1892 Edison ..209/128 ELECTROSTATICALLY CLASSIFYING 1,551,3978; 1925 Johnson..... ....209/ 129 1,355,477 10 1920 Howell ..209/129TONER PARTICLES 2,63,416 5/1953 Walkup et a1 ..209/ 127 X [72] Inventor:Bruce R. Robinson, Pittsford, NY.

Primary Examiner-Frank W. Lutter [73] Ass1gnee: Xerox Corporation,Rochester, NY. Assistant Emminer wimam Cuchlinski, Jr [22] Filed; Oct.20, 1969 Atmrney-Donald F. Daley, James J. Ralabate and James P.

OSullivan [21] Appl. No.: 867,739

[57] ABSTRACT [52] U.S.Cl ..209/129, 117/17.5, 118/610, Method andapparatus to electrostatically classify electro- 209/4 scopic tonerparticles according to size. Particles to be clas- [51] Int. Cl. ..B03c7/00 ifi ar ppor on a photoc n c ive rface nd electri- [58] Field ofSearch ..209/127-131, 3, lly arg d to One polarity. An electricallyconductive sur- 209/4; 1 17/ 1 7.5; 118/637; 101/1 14 face spaced apredetermined distance from the charged particles is electrically biasedby a potential of opposite polarity to [56] References Cited collecttoner particles which are attracted to the surface in response to themagnitude of the biasing voltage and the UNITED STATES PATENTSbetween-surface spacing.

2,428,224 9/1947 Johnson et a1. ..209/127 X 13 Claims, 6 Drawing FiguresELECTRICAL ELECTRICAL BIAS BIAS M POTENTIAL POTENTIAL PATENTEDMAY 16I972 3. 662 884 l l z 1 F 6. /a H @699; L! l 3 =I- if F/G. w

BIAS f POTENTIAL L I L FY F/G. /0

BIAS T POTENTIAL I I 1 F/G. /e I I I ELECTRICAL ELECTRICAL BIAS BIASPOTENTIAL POTENTIAL INVENTOR. BRUCE R. ROBINSON METHOD AND APPARATUSELECTROSTATICALLY CLASSIFYING TONER PARTICLES BACKGROUND OF THEINVENTION This invention relates to the electroscopic developingmaterials, and, in particular, to an improved method and apparatus forclassifying electroscopic particles.

More specifically, the invention relates to a classification method andapparatus whereby a very narrow size distribution of electroscopicparticles may be obtained by electrostatically attracting the particlesto an attraction member spaced therefrom a predetermined distance. Boththe particles to be classified and the attraction member are charged topredetermined voltage levels of opposite polarities.

Electroscopic materials, materials capable of retaining an electricalcharge on or in their surface, are extensively utilized in the field ofgraphic communication and especially in the science of xerography. As iswell known in the xerographic art, electroscopic developer materialcomprising toner powder and carrier material is used to develop a latentelectrostatic image on a photoreceptive surface. This image maysubsequently be transferred from the photoconductive surface to asupport material such as paper and permanently fixed thereto to form areproduction of the original document.

Various methods are utilized in the art to bring the electroscopic tonerparticles into contact with the latent electrostatic image and anexample is disclosed in Walkup, U.S. Pat. No. 2,618,551, wherein thetoner particles are carried to a photoconductive surface by a carriermaterial. The carrier material and toner particles are selected to havea predetermined triboelectric relationship whereby the toner istriboelectrically bonded to the carrier. The developer material,comprising the toner and carrier, is brought into contact with a latentelectrostatic image and the magnitude of the electrostatic force of thelatent image will overcome this triboelectric bond to physically removethe toner from the carrier and electrostatically bond it to the image.This type of latent electrostatic image development is known as cascadedevelopment. The carrier particles in cascade development are muchlarger than the toner and carry a plurality of toner particles to thephotoconductive surface. In addition to the cascade method ofdevelopment, other well-known development methods use a fur brush,magnetic particles simulating a brush, direct application of toner tothe image by means of a donor belt or simple dusting and the like.

Many different types of toner particles are utilized in the developmentof latent electrostatic images. One example of the range of sizes andcompositions of toner particles is disclosed in Carlson U.S. Pat. No.Re. 25,136, wherein an averagesize toner is indicated to be less than 20microns, preferably 5 to microns. However, toner particles of a sizeless than 5 microns or more than microns are utilized for certain latentimage development. In some commercially available toners the size of theindividual toner particles can range from less than 5 microns to asgreat as 50 microns. However, as higher speed and more efficientxerographic devices are developed, it is necessary to use tonerparticles of a more uniform size. For example, a more uniform sizedistribution will result in a more efficient transfer of toner particlesfrom the photoconductive surface to the support member thereby makingcleaning of the residual toner image left on the photoconductor drumafter transfer less difficult. Generally, image transfer is effected byan electrostatic field applied to the support material by a coronadischarge device or biased conductive roller device operativelycontacting the material on the side opposite the toner particles.However, if the toner particles comprising the developed image are ofnon-uniform sizes, an incomplete transfer of the toner will occur sincethe smaller particles will not be uniformly transferred under aparticular electrostatic field condition. Therefore, it becomesdifficult to adjust and select the electrostatic bias to be applied tothe support material to insure the complete image transfer if the tonerparticles are of a diverse range of sizes. It has been found to benearly impossible to effect a complete transfer of non-uniform sizedtoner to the support material. However, if toner is uniformly sized,transfer of the toner is significantly more efiicient as well asminimizing or eliminating the aforementioned difiiculty in removing aresidual toner powder image from the photoreceptor surface.

A further advantage in utilizing a uniform sized toner is better imagedevelopment characteristics. For example, when an image is developed bya process using a two component developer, the triboelectric bondbetween a toner and carrier is different for a toner of one size ascompared to toner of another size in relationship to the same sizecarrier as is well known. This variation in the triboelectric bondresults in a non-uniform range of attraction forces existing between thecarrier and toner, comprising developer, which prohibits control ofimage development. Beneficial results accrue from the use of uniformsized toner in every known development technique.

Numerous other advantages of using toner of substantially uniform sizedistribution should be apparent in nearly every step of the xerographicprocess. However, in order to attain a more uniform size toner it isnecessary that an efficient and effective size classification method andapparatus be utilized in selecting the proper toner. In the past,centrifugal force devices have been relied on to classify tonerparticles, but these results have produced a relatively broad sizedistribution. Further, centrifugal classifiers tend to be relativelylarge and expensive. The classification process and device of thisinvention, however, has been found to inexpensively and accuratelyachieve the desired result of a more narrow distribution of sizes of thetoner particles resulting in the separation of toner particles of asubstantially uniform size.

SUMMARY OF THE INVENTION It is, therefore, an object of this inventionto efficiently classify electroscopic particles according to size by animproved electrostatic method and apparatus.

A further object of this invention is to classify toner particles usedin the art of xerography within a narrow range of size distribution byselectively attracting charged particles of a predetermined size to anelectrically biased member.

There and other objects are attained in accordance with the presentinvention wherein there is provided a method and apparatus which isbased on phenomena that the larger size of uniformly charged particlescan be more readily attracted by a conductive electrode member biased byan electrical potential of opposite polarity than smaller particles. Byselecting the magnitude of the charge and bias and the distance betweenthe charged particles and conductive member, toner particles of acertain minimum size and greater may be attracted to the conductivemember while smaller particles are not attracted. Thereby, it ispossible to separate particles to desired size classifications byvarying the aforementioned various parameters according to desiredclassifications and obtain very narrow size distribution.

DESCRIPTION OF THE DRAWINGS Further objects of this invention, togetherwith additional features contributing thereto and advantage accruingtherefrom, will be apparent from the following description of oneembodiment of the invention when read in conjunction with theaccompanying drawings, wherein:

FIGS. la through e illustrate a diagrammatic analysis of the particleclassification of this invention.

FIG. 2 is a schematic view of one form of device for classifyingparticles according to this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring in particular to each ofthe FIGS. 1a through 12 there is illustrated the principles and theoryupon which the method and apparatus of this invention are based. Asdiscussed previously, the method of this invention utilizes anelectrostatic attraction of two oppositely charged elements to achieveclassification. For convenience of description, the polarities of theelectrostatic charges and bias potential illustrated in FIGS. 1c through1e and described below are only intended to be examples, and it iswithin the scope of the present invention to use opposite polaritiesthan as disclosed. To analyze this process in detail, reference is madein particular to FIG. la wherein a plate 2 is illustrated having auniform positive charge applied thereto. The plate is shown having atleast the surface being of a photoconductive insulating material as usedin the xerographic process, (e.g., see Eichler et al. U.S. Pat. No.2,945,434) but the plate could be of other materials capable ofretaining a charge. The positive charge may be deposited on the surfaceby any well-known process such as electrostatic charging by a coronadischarge device, induction charging device, or the like.

In FIG. lb the next step is illustrated wherein the toner particles tobe classified are placed in contact with the uniformly chargedphotoconductive plate and are electrostatically bonded thereto in asubstantially uniform layer. The toner particles 1 may be brought intocontact with the plate 2 by wellknown development methods such as theaforementioned cascade development. As shown in FIG. 1b the chargedtoner particles 1 of different sizes adhere to the oppositely chargedplate 2.

Referring now in particular to FIG. 10, there is illustrated the nextstep of the process. The developed plate is exposed to light from asuitable source 3 and a new positive electrostatic charge is applied tothe toner particles 1 on the plate by conventional means 4 of the typepreviously described. The toner particles on the photoconductive surfaceare exposed to light to dissipate much of the positive charge existingon the photoconductive plate through the conductive backing as shown inFIG. 10, but the charge is not removed in the interface areas betweenthe particles and surface, since illumination to those areas is blockedby the position of the toner. By subsequently applying a positive chargeof a sufficient magnitude upon the toner particles their polarity isreversed as further illustrated in FIG. 1c. These two steps of applyinga positive charge on the particles and exposing the plate to light maybe performed simultaneously as well as sequentially.

Referring to FIG. 1d, there is illustrated a conductive plate orelectrode 5 located a distance d from the photoconductive plate 2supporting the positive charged toner particles 1. The conductive plateis biased by an electrical potential to a polarity opposite from thepositively charged toner to establish an electrostatic field ofattraction between the particles 1 and the conductive surface 5. Themagnitude of the charge on the toner or the bias potential for a givendistance d controls the size distribution of the particles attracted tothe conductive plate 5. Such a result occurs because the electrostaticattraction force is a function of the unit area of the toner particles.By properly choosing the polarity and magnitude of the elec trical bias,particles of a predetermined size or greater can be selectivelyattracted to the conductive surface. It is also possible to vary thesize distribution of the particle attracted to the conductive plate byvarying the distance d between the toner and conductive plate 5 andmaintaining a constant bias potential on the conductive plate. FIG. 1eillustrates what occurs after transfer to the second plate 5 wherein thelarger positively charged toner particles are attracted to theconductive plate 5 and the smaller positively charged particles remainon the photoconductive surface 2.

In summarizing the process of the invention illustrated in FIGS. lathrough 1e, a photoconductive plate 2 is uniformly charged and the toner1 to be classified is placed in contact with the charged surface. Thephotoconductive plate and particles are exposed to light 3 and theparticles electrostatically charged to reverse their polarity. Theconductive plate 5, located a predetermined distance d from theparticles, is electrically biased to a predetermined magnitude of apolarity opposite to the charge on the toner particles, whereby theparticles of a predetermined size distribution are attracted to theconductive surface. This predetermined size distribution depends onforce of the electric field which is a function of the variables of theconductive surface biasing potential magnitude and the initial charge onthe particles as well as inversely to the distance d. It is alsopossible to further classify the particles collected on the conductiveplate by placing a second conductive surface adjacent the firstconductive surface. This second surface is connected to a different biasof like polarity and thereby attracts particles of a second largerpredetermined size from the first conductive surface. A plurality ofsuch surfaces could be utilized whereupon it would be possible tocollect on each surface the desired size classification.

For an example of the process of this invention, the photoconductiveplate 2 was positively charged to 600 volts. The plate was thendeveloped by a uniform layer of toner particles to be classified on thecharged plate. The plate 2 was then exposed and the particles 1 chargedpositively by a corotron applying a 2/ma/inch current at 3inches/second. An aluminum plate 5 was supported 7 mils from the chargedparticles and a negative bias of approximately 1,000 volts was appliedto the metal. Approximately to 98 percent of the particles transferredto the aluminum plate were 10 microns or greater. To further classifythe particles of 10 microns or greater collected by the first plate 5, asecond plate biased by approximately 1,000 volts was positioned 7 milsfrom first plate 5 resulting in a still larger range of particles beingtransferred to the second plate whereupon particles in the range of 10to 20 microns remained on the first conductive plate 5. A larger rangeof particles was transferred to the second conductive plate, because thelevel of charge on the particles changed in the first transfer from thephotoconductive plate 2 to the plate 5. However, depending on the chargeon the collected particles on the first conductive plate 5 and thedesired range of particle sizes subsequently to be left thereon, themagnitude of the bias on the second conductive plate may be selected tobe a different value than the bias applied to the plate 5. From theseresults it can be seen that the process produces very effectiveclassification of toner particles according to size. As is apparent fromthe foregoing, different magnitudes of voltage and distance may beemployed according to desired classification.

Referring now in particular to FIG. 2, an embodiment of the device forclassifying the particles according to the invention is illustrated. Adrum 20 is rotatably mounted on suitable bearings (not shown) and isdriven by a motor M in a counterclockwise direction. The drum includes astandard xerographic surface 21 comprising a conventionalphotoconductive insulating material. Mounted adjacent to the top of thedrum is a charging device 22 for depositing a uniform electrostaticcharge on the photoconductive surface. Charging device 22 may be anymeans capable of uniformly charging an insulating surface, as forexample, a conventional corotron. Adjacent the left side of the drum isa conventional development device 23 such as the aforementioned cascadesystem for bringing particles to be classified in contact with thephotoconductive surface. Other developing means, such as a brush,magnetic, or a simple dusting device, may be utilized for development.

Located next in the path of drum surface movement is a charging device24 for placing a charge on the developed photoconductive surface and anexposure device 25 for dissipating much of the surface charge on thephotoconductive surface of the drum. A conductive drum 30 is rotatablymounted adjacent the drum 20 with the surface of the conductive drumlocated a predetermined distance d from the photoconductive surface. Theexact predetermined distance is selected according to desired sizeclassification of the particles. Drum 30 is slidably supported from abracket 31 to allow adjustment of the distance between the surfaces. Thedrum 30 may be made of any electrically conductive material such asaluminum and is connected to an electrical bias potential of oppositepolarity from the charged particles. The conductive drum also has anadjacently mounted charging device 32 which may be a corotron as shownor the like, to charge the toner particles attracted to the conductivedrum. By charging the collected particles to a predetermined level, atransfer of a' different size range of particles may be accomplishedfrom the first conductive drum to a second conductive drum to allow amore narrow classification of size to be discussed later. The firstconductive drum includes a collection means 33 for collecting theattracted particles and a brush 34 to clean the surface.

The second conductive drum 40, preferably metal, is similar to the firstconductor drum and is rotatably mounted in a suitable bearing. Anelectrical bias potential having the same polarity as that on theconductive drum 30 is applied to this second drum. By selecting asuitable magnitude for both the bias on the second drum and the chargeplaced on the attracted particles of the first conductor drum, anotherrange of particle size will be attracted from drum 30 to the second drum40 to further classify the toner. It is understood, however, that thedistance between the two conductor drums is a variable which also mustbe considered. Accordingly, the second conductive drum includes distanceadjustment means (not shown) such as an adjustable bracket. Therefore,according to this invention it is clear that a plurality of suchconductive drums could be mounted in tandem to achieve a very preciseclassification of toner.

Collection means 41, adjacent the second drum, illustrates an example ofhow the transferred particles are collected. A nozzle 42, connected to apressurized air source 43 to direct an air jet onto the surface of thedrum 40, loosens the collected particles on the drum surface and anexhaust means 44 removes the loosened toner. Exhaust means 44 is coupledto a particle container 45 for collecting the toner. Other collectionmeans could be utilized in the device of this invention and if aplurality of conductive drums were employed in the device, each drumwould include such a collection means.

in operation, the photoconductive surface is charged by the chargingdevice 22 with, for example, a positive electrostatic charge. Thecharged surface is rotated past the development means 23 where particlesto be classified come in contact with and cling to the photoconductivesurface. The developed area then moves past the second charging means 24and exposure means 25 whereby a positive charge is placed on theparticles, and much of the charge on the photoconductive surface isdissipated because of the light exposure of the photoconductive surface.The surface of the photoconductor drum bearing the charged particlesthen rotates to a position adjacent the surface of the negatively biasedconductive drum 30. By electrostatic attraction, particles larger than aprecalculated size transfer to the conductive drum 30, while theparticles smaller than this size remain on the photoconductive surface.The particles transferred to the conductive drum rotate past thecharging device 32 and are again positively charged. These transferredparticles are rotated to a position in operative contact with the secondconductive drum, which is also negatively biased. The second drumremoves another predetermined range of sizes from the first conductivedrum. The predetermined size of the toner particles attracted to thesecond drum is greater than the particles attracted to the firstconductive drum selectively excluding a portion of the particles whichwill remain on the first surface. In operation, a plurality of theseconductive surfaces would provide a very close distribution of the sizeof the toner. Collection means for each drum operates to remove thecollected and sorted toner particles.

it has been found that when the conductive drum 30 is negatively biasedin the range of 250-],000 volts classification is effected for manyconventional toners. If more than one conductive drum is utilized, theneach drum is negatively biased in the range of 250-1,000 volts. Therange of separation of the charged particles and the surface of thefirst conductive drum 30 for the above bias has been found to bepreferably 5 to 20 mils. Each additional conductive drum is separatedfrom the adjoining one a similar order of distance. The device canemploy voltage polarities and magnitudes and spacing ranges other thanthose indicated here depending on the range of sizes of the particles tobe classified as well as the size distribution desired.

Modifications of the aforedescribed method and apparatus are within thescope of this invention. For example, the polarity of charges and biasdisclosed in the method and apparatus of this invention could beopposite than described, as long as the polarity of the charge on theparticles is opposite to the bias on the collecting member. Also, theconductive attraction member could be of an insulating material havingan electrostatically charged surface. It is further within the scope ofthis invention to classify any electroscopic particles as well as thetoner particles used in the xerographic process. Moreover, the form ofthe drums used in the description of the embodiment of FIG. 2 could bemodified to use various alternatives, as for example, a web or platemeans.

While this invention has been described with reference to the structureand process disclosed herein, it is not intended to be confined to thedetails set forth or the specific environment set forth. Therefore, thisapplication is intended to cover such modifications or changes as maycome within the purposes of the invention or the scope of the followingclaims.

What is claimed is:

l. A method for classifying electroscopic particles according to sizecomprising the steps of placing an electrostatic charge of a firstpolarity upon a photoconductive insulating surface, dusting said surfacewith electroscopic particles to be classified charged to a secondpolarity opposite the first by charging means so that the particlescling to said surface,

exposing said surface to light to dissipate the charge thereon,

placing a second predetermined charge of said first polarity upon saidparticles on said surface,

and applying a predetermined bias potential of the second polarity upona conductive surface located a predetermined distance from saidphotoconductive surface to attract particles of a predetermined size tosaid conductive surface for classification.

2. The method of claim 1 further comprising the step of collecting theparticles attracted to said conductive surface.

3. The method of claim 1 wherein said step of exposing and said step ofplacing said second charge are performed simultaneously.

4. The method of claim I further comprising the step of placing a secondbias potential of a different magnitude than said predetermined biaspotential upon a second conductive surface located a predetermineddistance from said first conductive surface whereas particles of asecond or more range of sizes of particles are attracted to said secondor more conductive surfaces from said first conductive surface for afurther classification of said particles.

5. The method of claim 1 further comprising the step of varying themagnitude of one of said second charge or said bias potential toselectively vary the range of sizes of said particles attracted to saidconductive surface.

6. The method of claim 1 further comprising the step of varying thedistance between said photoconductive surface and said conductivesurface to alter the range of sizes of said particles attracted to saidconductive surface.

7. A method of classifying according to size electroscopic tonerparticles composed of the same material comprising charging theparticles to provide charged particles of a first charge polarity,

forming a particle layer by electrically attracting charged particles ofthe first polarity to a support member electrically biased to a firstpotential,

depositing electrostatic charge of a second polarity opposite the firstpolarity onto the particle layer, and

separating larger particles from the particle layer by steps includingspacing an electrically conductive separation member a predetermineddistance from the particle layer and coupling an electrical bias of asecond potential of a different magnitude than said first potential tothe separation member whereby the magnitude of the electrostatic chargeon the particles, the difference between the first and second biaspotentials and the spacing between the support and separation membersestablish the size particle separated from the particle layer.

8. The method of claim 7 wherein the surface of the support member onwhich the particle layer is formed is electrically conductive.

9. The method of claim 7 wherein the surface of the support member onwhich the particle layer is formed includes a photoconductive insulatingmaterial and further including the step of exposing particle layer tolight simultaneously with the depositing of electrostatic charge on theparticle layer.

10. The method of claim 7 wherein the surface of the support member onwhich the particle layer is formed includes a photoconductive insulatingmaterial and further including the step of exposing the particle layerto light subsequent to depositing electrostatic charge on the particlelayer.

11. The method of claim 7 further including classifying the particlesseparated from the particle layer to the separation member by stepsincluding moving the separated particles on the separation member fromthe vicinity of the support member,

spacing a third electrically conductive member a predetermined distancefrom the separated particles on the separation member, and

coupling an electrical bias of a third potential to the third memberwhereby the magnitude of the charge on the separate particles, thedifference between the second and third potentials and the spacingbetween the separation and third members establish the size particleattracted to the third member for further classification of theparticles.

12. The method of claim 11 further including depositing electrostaticcharge onto the separated particles on the separation member prior tospacing the third member adjacent the separated particles.

13. Apparatus for classifying according to size electroscopic tonerparticles composed of the same material comprising a xerographic platesurface including a photoconductive surface said plate being coupled toan electrical bias of a first potential, first charging means fordepositing electrostatic charge of a first polarity on a surface of saidsupport member,

development means for forming a particle layer of charged tonerparticles of a second polarity opposite the first on the charged surfaceof said support member,

second charging mean for depositing electrostatic charge of the secondpolarity on said particle layer formed on the support member,

exposure means for exposing said charged particle layer to light and anelectrically conductive separation member coupled to an electrical biasof a second potential and adapted to be spaced a predetermined distancefrom a charged and exposed particle layer whereby the magnitude of thecharge of second polarity, the difference between the first and secondpotentials and the spacing between the support and separation membersestablish the size particle separated from the particle layer on thesupport member.

1. A method for classifying electroscopic particles according to sizecomprising the steps of placing an electrostatic charge of a firstpolarity upon a photoconductive insulating surface, dusting said surfacewith electroscopic particles to be classified charged to a secondpolarity opposite the first by charging means so that the particlescling to said surface, exposing said surface to light to dissipate thecharge thereon, placing a second predetermined charge of said firstpolarity upon said particles on said surface, and applying apredetermined bias potential of the second polarity upon a conductivesurface located a predetermined distance from said photoconductivesurface to attract particles of a predetermined size to said conductivesurface for classification.
 2. The method of claim 1 further comprisingthe step of collecting the particles attracted to said conductivesurface.
 3. The method of claim 1 wherein said step of exposing and saidstep of placing said second charge are performed simultaneously.
 4. Themethod of claim 1 further comprising the step of placing a second biaspotential of a different magnitude than said predetermined biaspotential upon a second conductive surface located a predetermineddistance from said first conductive surface whereas particles of asecond or more range of sizes of particles are attracted to said secondor more conductive surfaces from said first conductive surface for afurther classification of said particles.
 5. The method of claim 1further comprising the step of varying the magnitude of one of saidsecond charge or said bias potential to selectively vary the range ofsizes of said particles attracted to said conductive surface.
 6. Themethod of claim 1 further comprising the step of varying the distancebetween said photoconductive surface and said conductive surface toalter the range of sizes of said particles attracted to said conductivesurface.
 7. A method of classifying according to size electroscopictoner particles composed of the same material comprising charging theparticles to provide charged particles of a first charge polarity,forming a particle layer by electrically attracting charged particles ofthe first polarity to a support member electrically biased to a firstpotential, depositing electrostatic charge of a second polarity oppositethe first polarity onto the particle layer, and separating largerparticles from the particle layer by steps including spacing anelectrically conductive separation member a predetermined distance fromthe particle layer and coupling an electrical bias of a second potentialof a different magnitude than said first potential to the separationmember whereby the magnitude of the electrostatic charge on theparticles, the difference between the first and second bias potentialsand the spacing between the support and separation members establish thesize particle separated from the particle layer.
 8. The method of claim7 wherein the surface of the support member on which the particle layeris formed is electrically conductive.
 9. The method of claim 7 whereinthe surface of the support member on which the particle layer is formedincludes a photoconductive insulating material and further including thestep of exposing particle layer to light simultaneously with thedepositing of electrostatic charge on the particle layer.
 10. The methodof claim 7 wherein the surface of the support member on which theparticle layer is formed includes a photoconductive insulating materialand further including the step of exposing the particle layer to lightsubsequent to depositing electrostatic charge on the particle layer. 11.The method of claim 7 further including classifying the particlesseparated from the particle layer to the separation member by stepsincluding moving the separated particles on the seParation member fromthe vicinity of the support member, spacing a third electricallyconductive member a predetermined distance from the separated particleson the separation member, and coupling an electrical bias of a thirdpotential to the third member whereby the magnitude of the charge on theseparate particles, the difference between the second and thirdpotentials and the spacing between the separation and third membersestablish the size particle attracted to the third member for furtherclassification of the particles.
 12. The method of claim 11 furtherincluding depositing electrostatic charge onto the separated particleson the separation member prior to spacing the third member adjacent theseparated particles.
 13. Apparatus for classifying according to sizeelectroscopic toner particles composed of the same material comprising axerographic plate surface including a photoconductive surface said platebeing coupled to an electrical bias of a first potential, first chargingmeans for depositing electrostatic charge of a first polarity on asurface of said support member, development means for forming a particlelayer of charged toner particles of a second polarity opposite the firston the charged surface of said support member, second charging mean fordepositing electrostatic charge of the second polarity on said particlelayer formed on the support member, exposure means for exposing saidcharged particle layer to light and an electrically conductiveseparation member coupled to an electrical bias of a second potentialand adapted to be spaced a predetermined distance from a charged andexposed particle layer whereby the magnitude of the charge of secondpolarity, the difference between the first and second potentials and thespacing between the support and separation members establish the sizeparticle separated from the particle layer on the support member.